1. Field of the Invention
The present invention concerns a method to determine the phase position of a magnetization, as well as a magnetic resonance system to implement such a method.
2. Description of the Prior Art
Magnetic resonance tomography is a widespread method to acquire medical image data of a subject to be examined. The subject to be examined is introduced into an optimally homogenous, static magnetic field (B0 field). To acquire image data, magnetic gradient fields are applied and electromagnetic radio-frequency (RF) pulses are radiated that excite nuclear spins that precess around the magnetic fields. The magnetic flux density of the RF pulses is often designated B1. Upon excitation with RF pulses, the direction of the magnetization that is originally aligned parallel to the B0 field is tilted relative to this by a predetermined angle (flip angle). The flip angle depends both on the radiation duration of the RF pulse and on the B1 field strength. For example, given a flip angle of 90°, a transverse magnetization (perpendicular to the B0 field) can be generated, the decay of which is subsequently acquired as a magnetic resonance signal. To generate qualitatively high-grade image data, it is consequently desirable to obtain an optimally uniform deflection of the magnetization across a slice of the subject to be examined.
With the use of conventional RF pulses, B1 inhomogeneities within a slice can occur, in particular due to an inhomogeneous penetration behavior in dielectric media (for example human or animal tissue). This leads to a non-uniform signal intensity and thus to a non-uniform contrast, particularly at high field strengths. In order to solve this problem, various B1-insensitive, selective RF pulses have been developed. A precise slice selection can be achieved with composite adiabatic pulses. For specific imaging sequences, however, the use of adiabatic RF pulses is problematic since the magnetization generated by the pulses has an unknown phase position. It is necessary to know the phase position for the radiation of the refocusing pulses, in particular for multi-echo sequences (for example a turbo spin echo sequence).
The problem of the unknown phase position is solved in a conventional magnetic resonance method by replacing an adiabatic excitation pulse of an adiabatic excitation sequence with a conventional 90° excitation pulse, as described in Conolly et al., Magn Reson Med 1991; 18(1):28-38. The problem of the B1 field inhomogeneity occurs, however, for this first pulse, which leads to an inhomogeneous exposure of the volume to be examined, and therefore in turn to a non-uniform signal intensity. An additional approach is the prediction of the RF-dependent phase by simulation of the Bloch equations for the composite adiabatic RF pulses. However, the phase is sensitive to B0 inhomogeneities. If this technique is used with turbo/fast spin echo sequences, the signal decreases. The use of adiabatic RF pulses for a multi-echo imaging sequence is thus likewise not enabled by this method. In particular, for the fulfillment of a Car-Purcell-Meiboom-Gil (CPMG) condition in a multi-echo imaging sequence, it is necessary to know the orientation of the magnetization after excitation since this should coincide with the magnetic field axis of refocusing pulses.